Can we have a quick vote about who prefers the current option 1:

(keyword-lambda (a b (c d e)) <lambda body ...>)

(keyword-call foo 1 2 (e 5 c 3 d 4))

And who would prefer option 2 where all of these are supported:

(keyword-lambda (a b :key c d e) <lambda body ...>)

(keyword-call foo 1 2 e: 5 c: 3 d: 4)     ; In all Schemes
(keyword-call foo 1 2 :e 5 :c 3 :d 4)     ; In all Schemes
(keyword-call foo 1 2 #:e 5 #:c 3 #:d 4)  ; In Schemes that have #:

Option 1 can be implemented with syntax-rules and tail patterns.

Option 2 needs syntax-case, explicit-renaming, or define-macro. It can
be implemented equally well in Schemes that have native read syntax for
keywords, and Schemes that don't. The trick is to treat a symbol
starting or ending with a colon as a keyword.